Lenses of ferruginous quartzite in an area of mica gneiss east of Igawa, Tanganyika, are considered to have acted as competent bodies during deformation of the gneiss. The quartzite probably represents beds originally composed of chert, whose resistance to both deformation and migmatization now accounts for differences in strike between the quartzite and the gneiss, and also for the unusual course of the Mbarali river.

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